1. Field of the Invention
Technology of electrical superconductors. The importance of materials which have superconducting properties has greatly increased very recently. The discovery of new superconducting materials, in particular of the rare earth/Ba/Cu/O type, led to a considerable expansion of the possible applications of superconductors, since these materials already become superconducting at temperatures above 50 K.
The invention relates to the further development and improvement of products made from a ceramic high temperature superconductor, wherein the needs of large-scale industrial production are to be taken into account.
In particular, the invention relates to a process for the production of a polycrystalline, crystal-oriented surface layer on a substrate or an interlayer between two substrates, where said layer consists of a ceramic high temperature superconductor based on (Y,RE) Ba.sub.2 Cu.sub.3 O.sub.6.5+y, where RE=rare earth metal and 0&lt;y&lt;1.
2. Discussion of Background
Among the oxide high temperature superconductors, the compounds containing yttrium, barium and copper appear to be particularly promising. In contrast to the classical metallic or intermetallic superconductors, however, their brittleness, the difficulty of processing them, and above all their deficient current conductivity (low critical current density j.sub.c) are obstacles to their use. The oxide compound YBa.sub.2 Cu.sub.3 O.sub.6.5+y (0&lt;y&lt;1) is a superconductor with a critical temperature T.sub.c =93 K, when it is present in the orthorhombic modification and y is about 0.4-0.5. If a monocrystalline thin layer (d=50-500 nm) is produced by vapor deposition or cathode sputtering, maximum critical current densities j.sub.c .apprxeq.5.times.10.sup.6 A/cm.sup.2 (at 77 K and magnetic field H=0) parallel to the plane of the layer are found when the layer is oriented with its orthorhombic c-axis perpendicular to the plane of the layer. This orientation is obtained when a single crystal of SrTiO.sub.3 is used as the substrate with its surface oriented such that the &lt;100&gt; direction of its cubic structure is perpendicular to the surface. Apart from this, the substrate temperature and atmosphere in the coating apparatus must be suitably chosen. On the other hand, critical current densities of at most only j.sub.c .apprxeq.1000 A/cm.sup.2 (T=77 K, H=0) were obtained for polycrystalline, massive ceramics, even under optimum conditions of production. The weak coupling of the superconductor currents over grain boundaries has long been known as a cause of this severe reduction of j.sub.c in polycrystalline ceramics, which severely restricts technical applications. It is further known that in this respect the grain boundaries perpendicular to the c-axis have a particularly unfavorable action, while the grain boundaries perpendicular to the a- or b-axis can carry much greater superconductor currents. Hence there has been no lack of research to produce ceramics, and in particular polycrystalline layers of the above-named material, whose crystals are aligned as completely as possible so that there are no grain boundaries in the direction of the superconducting current which severely reduce the current. All of the crystallites must then have their c-axes perpendicular to the substrate surface, while the a- and b-axes can be arbitrarily oriented. Indeed, layers have now already been produced whose crystallites were partially oriented in this manner, but up to now only disappointingly low critical current densities were obtained, at most 70 A/cm.sup.2, i.e., less than 1/10 of that found in the best unoriented ceramics. This inadequate result can be partially explained by the use of unsuitable substrates which react in an undesired manner on sintering of the superconducting substance and which contaminate the grain boundaries with foreign phases.
The following literature references are cited with respect to the state of the art:
F. Kloucek, W. E. Rhine, H. K. Bowen, "Thick films of YBa.sub.2 Cu.sub.3 O.sub.x fabricated from colloidal solutions", Internat. Conf. on High-T Superconductors, Interlaken 1988 PA1 P. Murugaraj et al., "Preparation of highly oriented Polycrystalline YBa.sub.2-y Cu.sub.3 O.sub.x Superconductors", Solid State Communciations, vol. 66, No. 7, 1988, pp. 735-738 PA1 Tadashi Takenaka, Hideki Noda, Atsuhiko Yoneda and Koichiro Sakata, "Superconducting Properties of Grain-Oriented YBa.sub.2 Cu.sub.3 =.sub.7 -x Ceramics", Japanese Journal of Applied Physics, Vol. 27, No. 7, 1988, pp. L1209-L1212 PA1 Junji Tabuchi and Kazuaki Utsumi, "Preparation of superconducting Y-Ba-Cu-O thick films with preferred c-axis orientation by a screen-printing method", Appl. Phys. Lett. 53(7), 15 August 1988
J. T. Markert, B. D. Dunlap, and M. B. Maple, "Magnetism, Superconductivity, and Chemical Substitutions in YBa.sub.2 Cu.sub.3 O.sub.7-.delta. ", MRS Bulletin/January 1989.
The superconducting ceramic layers produced by the known processes are insufficient by far for the present requirements. There therefore exists a great need for improvement and further development of such layers.